104
C. Variations of Stable Isotope Ratios in Nature
isotopic fractionations do not preserve a record of the highest temperature of metamorphism (1(max)). Therefore, there has been retrogression
down to some temperature below 1(max) at which equilibrium may again
be frozen in. This is probably because subsequent to reaching 1(max) ,
some minerals in the rock continued to exchange with one another,
down to some lower temperature."
Another important point of oxygen isotope determinations on metamorphic rocks is that there is evidence for interaction between the metamorphic mineral assemblages and a fluid phase permeating the rocks at
the time of metamorphism. TAYLOR and COLEMAN (1968) have pointed
out that the result of this interaction is that any mineral in equilibrium
with an isotopically homogeneous pore fluid would attain the same 18 0/
16 0 ratio irrespective of rock type as long as the temperature is constant.
Quartz for instance should attain the same oxygen isotope composition in
all adjoining meta-sedimentary or meta-igneous rocks, because of equilibration with the same pore fluid.
The effect of dehydration reactions upon the oxygen isotope composition of pelitic rocks can be studied by examining the variations of the
180-conten t across metamorphic zones. SHIEH and TAYLOR ( 1969 a) have
found that dehydration reactions would not produce any appreciable
oxygen isotopic changes at least during contact metamorphism.
Metamorphism has been classified into several different types, two
most important of which are:
1) Regional metamorphism, occurring on a regional scale in areas up
to thousands of square kilometers.
2) Contact metamorphism, taking place in heated rocks bordering
magmatic intrusions.
1. Regional Metamorphism
When considering regional metamorphic rocks, the most striking
feature is that the 180rO ratios of pelitic rocks tend to decrease with
increasing metamorphic grade. The Jl80-value in pelitic rocks of low
grade (chlorite zone) is close to the values typical of shales (15 to 18%0). It
decreases more or less steadily through the biotite, garnet, and staurolite-kyanite zones to values in the range 9 to 13%0 for high-grade sillimanite schists. A comparison of Jl80-values of regional with contact metamorphic rocks is shown in Fig. 36.
2. Contact Metamorphism
Because the oxygen isotopic composition of igneous rocks is quite
different from that of sedimentary and low-grade metamorphic rocks,
studies on the variations of oxygen isotopes in the vicinity of an intrusive
C. Variations of Stable Isotope Ratios in Nature
isotopic fractionations do not preserve a record of the highest temperature of metamorphism (1(max)). Therefore, there has been retrogression
down to some temperature below 1(max) at which equilibrium may again
be frozen in. This is probably because subsequent to reaching 1(max) ,
some minerals in the rock continued to exchange with one another,
down to some lower temperature."
Another important point of oxygen isotope determinations on metamorphic rocks is that there is evidence for interaction between the metamorphic mineral assemblages and a fluid phase permeating the rocks at
the time of metamorphism. TAYLOR and COLEMAN (1968) have pointed
out that the result of this interaction is that any mineral in equilibrium
with an isotopically homogeneous pore fluid would attain the same 18 0/
16 0 ratio irrespective of rock type as long as the temperature is constant.
Quartz for instance should attain the same oxygen isotope composition in
all adjoining meta-sedimentary or meta-igneous rocks, because of equilibration with the same pore fluid.
The effect of dehydration reactions upon the oxygen isotope composition of pelitic rocks can be studied by examining the variations of the
180-conten t across metamorphic zones. SHIEH and TAYLOR ( 1969 a) have
found that dehydration reactions would not produce any appreciable
oxygen isotopic changes at least during contact metamorphism.
Metamorphism has been classified into several different types, two
most important of which are:
1) Regional metamorphism, occurring on a regional scale in areas up
to thousands of square kilometers.
2) Contact metamorphism, taking place in heated rocks bordering
magmatic intrusions.
1. Regional Metamorphism
When considering regional metamorphic rocks, the most striking
feature is that the 180rO ratios of pelitic rocks tend to decrease with
increasing metamorphic grade. The Jl80-value in pelitic rocks of low
grade (chlorite zone) is close to the values typical of shales (15 to 18%0). It
decreases more or less steadily through the biotite, garnet, and staurolite-kyanite zones to values in the range 9 to 13%0 for high-grade sillimanite schists. A comparison of Jl80-values of regional with contact metamorphic rocks is shown in Fig. 36.
2. Contact Metamorphism
Because the oxygen isotopic composition of igneous rocks is quite
different from that of sedimentary and low-grade metamorphic rocks,
studies on the variations of oxygen isotopes in the vicinity of an intrusive